Det här låter mycket som Stepsons of Terra av Robert Silverberg.
Jag kan inte hitta mycket information på nätet ( en recension här ). Från det jag minns handlar det om att någon reser till jorden från en avlägsen koloni för att söka hjälp. Det handlar också om kloning via tidsresor.
... en man vars planet skulle bli attackerad av utlänningar om ett par år. För att förhindra att planeten förstördes skickades han tillbaka till jorden (långt borta) för att få hjälp.
Already four worlds had fallen to the Klodni since the aliens had begun their campaign of conquest. The timetable said they would reach Corwin within the next decade.
...
Ewing shuddered as his tiny ship spiralled Earthward, bobbing in the thickening layers of atmosphere. Earth will save her colonies from conquest.
Jorden var en planet full av akademiker och små människor fokuserade bara på filosofi och saker av den sorten.
The more Ewing read, the less confident he became that he would find any aid for Corwin here. Earth had become a planet of gentle scholiasts, it seemed; was there anything here that could serve in the struggle against the advancing Klodni?
... huset ansågs vara i en annan "tid". Han hade en speciell nyckel för att komma in i huset
Myreck held his key in front of him, stpped into the lot and groped briefly in mid-air, as if searching for an invisible keyhole. And indeed he seemed to find it; the key vanished for three-quarters of its length.
...
Myreck smiled. 'The house is three microseconds out of phase with the rest of the street. It always exists just a fraction of an instant in Absolute Past, not enough to cause serious temporal disturbance but enough to conceal it from our many enemies.
kloning själv
Ewing kidnappas av Sirians (en annan koloni). Han räddas av en förtäckt främling. När han visar tidsmaskinen inser han att han var räddare och upprepar processen.
He realized that he had several choices. He could walk out of the hotel room and leave Ewing-sub-two to his own devices, in which case, in the normal flow of events, Ewing-sub-two wuold awaken, be taken to Myreck's, request to see the time machine, and in due course travel back to this day to become Ewing-sub-one, rescuing a new Ewing-sub-two.
... skicka utlänningarna 4 miljarder år tidigare, där ingenting väntar på dem.
He wondered briefly how the puzzled Klodni would react when they found themselves in the midst of a trackless void, without stars, without planets.
...
Ewing had hurled the Klodni fleet five billion years into the past. He shuddered at the thought, and turned his tiny ship homeward, to Corwin.